1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of changing, depending on a size of sheets, a pressure contact force for bringing the sheets into pressure contact with a sheet feeding roller.
2. Description of the Related Art
A conventional image forming apparatus, such as a copying machine, a laser beam printer (LBP), or a facsimile, includes a sheet feeder for feeding sheets one by one from a sheet feeding cassette storing multiple sheets. In general, the conventional image forming apparatus includes a sheet feeding cassette mountably installed in an apparatus main body, and a pickup roller for automatically feeding sheets stored in the sheet feeding cassette to an image forming portion. The sheet feeding cassette may be provided with an intermediate plate (sheet stacking plate) on which sheets are stacked. The intermediate plate is provided in a liftable manner. When the sheet feeding cassette is installed in a predetermined installation position in the image forming apparatus main body, the intermediate plate is pushed up by a lifter mechanism at the installation position to be rotated upward.
When the intermediate plate is rotated upward, the uppermost sheet of the sheets stacked on the intermediate plate is pressed against the pickup roller. The pickup roller is also moved up when the intermediate plate is further moved upward. After that, the intermediate plate is stopped when the pickup roller is moved up to a predetermined position. The intermediate plate stops at a position for applying an appropriate sheet feeding pressure on an upper surface of the sheets by the pickup roller when feeding the sheets. The intermediate plate is stopped at such a position, to thereby allow the sheets to be reliably fed. With this configuration, the uppermost sheet is always pressed against the pickup roller to be fed even in a case where the number of sheets is small.
FIG. 21 is a diagram for illustrating an example of the conventional lifter driving mechanism described above. In FIG. 21, a sheet feeding cassette 110 may be mountably installed in an apparatus main body. The sheet feeding cassette 110 is provided with a sheet stacking plate 111 on which sheets S are stacked. The sheet stacking plate 111 is provided as being rotatable in a vertical direction. Provided below the sheet stacking plate 111 is a pressing lever 112 serving as a lift member for pushing up the sheet stacking plate 111 toward a sheet feeding roller 102. The pressing lever 112 is coupled to a pressing arm 113 at the back of the apparatus main body, and connected to a lifter rack 115 via a pressure spring 114 serving as a helical extension spring. A cassette gear 116 is provided inside the sheet feeding cassette 110, and functions as a pinion gear for the lifter rack 115. A driving force transmission gear 104 is provided to the apparatus main body, and serves as a driving unit for driving the pressing lever 112. The driving force transmission gear 104 receives a driving force transmitted by a motor (not shown), to thereby transmit the driving force to the cassette gear 116.
In the sheet feeding apparatus configured as described above, when the sheet feeding cassette 110 is installed in the apparatus main body, the cassette gear 116 provided to the sheet feeding cassette 110 is engaged with the driving force transmission gear 104 provided to the apparatus main body. When a detecting unit (not shown) detects that the sheet feeding cassette 110 is installed, the motor (not shown) is driven. Along with the driving of the motor, the driving force transmission gear 104 is rotated, and the lifter rack 115 starts to move in a direction indicated by the arrow P via the cassette gear 116. As a result, the sheet stacking plate 111 is moved up via the pressure spring 114 coupled to the lifter rack 115, the pressing arm 113, and the pressing lever 112, and the sheets S on the sheet stacking plate 111 are brought into pressure contact with the sheet feeding roller 102. After that, the motor (not shown) is stopped after being rotated for a predetermined number of times. At this time, the pressure spring 114 serves as a pressing unit for applying a sheet feeding pressure to the sheet feeding roller 102 and the sheet S.
After that, when the sheets S are sequentially fed and the number of sheets S on the sheet stacking plate 111 decreases, the pressing arm 113 and the pressing lever 112 are turned in a CCW direction by the pressure spring 114, so as to push up the sheet stacking plate 111. During the sheet feeding operation, the position of the lifter rack 115 is fixed, and hence the acting length of the pressure spring 114 is reduced when the pressing arm 113 is turned by an increased amount as the number of sheets on the sheet stacking plate 111 decreases. At this time, a rate of decrease in weight of the sheets S on the sheet stacking plate 111 and the spring force of the pressure spring 114 which is reduced in acting length may be balanced, to thereby apply a constant sheet feeding pressure regardless of the varying amount of stacked sheets. As a result, double feeding which is likely to occur when the sheet feeding pressure is too high or nonfeeding which is likely to occur when the sheet feeding pressure is too low may be prevented, to thereby attain stable sheet feeding performance regardless of the varying amount of stacked sheets.
An apparatus provided with the lifter driving mechanism which moves up the sheet stacking plate by the pressing lever turned by the motor as described above is known to excel in operability when attaching and detaching the sheet feeding cassette. For example, conventionally, there is an image forming apparatus with a configuration in which a pressing force to be applied to the sheet stacking plate is generated as the sheet feeding cassette is pushed into the image forming apparatus main body. With this configuration, the sheet feeding cassette needs to be pushed against the pressing force to be installed, which leads to a problem that a larger resistance force is generated when installing the sheet feeding cassette which has a larger number of sheets stacked on the sheet stacking plate. In particular, in the case of an image forming apparatus in which the sheet feeding cassette is installed and detached in a direction orthogonal to a sheet feeding direction, the sheet feeding cassette has an asymmetric structure in which a sheet supplying portion is disposed on one of the right side and the left side of the sheet feeding cassette. As a result, the resistance forces on the right and the left of the sheet feeding cassette are different from each other, which makes the installation and detachment of the sheet feeding cassette even more difficult.
According to the lifter driving mechanism, the pressing force is exerted by pushing up the sheet stacking plate after the sheet feeding cassette is installed in the image forming apparatus main body. Accordingly, there is no resistance force generated when installing the sheet feeding cassette, and hence the sheet feeding cassette may be installed smoothly. Even in the case of the image forming apparatus in which the sheet feeding cassette is installed and detached in a direction orthogonal to the sheet feeding direction, the sheet feeding cassette may be installed and detached smoothly without no one-sided resistance force to be generated. Japanese Patent Application Laid-Open No. 2006-56685 discloses a sheet feeder provided with the above-mentioned lifter driving mechanism, which includes a sheet feeding pressure control unit for controlling the sheet feeding pressure according to the sheet size. For example, the sheet feeder illustrated in FIG. 21 is provided with a sheet feeding pressure control unit, in which an amount of displacement of the lifter rack 115 is changed depending on the sheet size so that the acting length of the pressure spring 114 is changed, to thereby control the sheet feeding pressure. Due to the sheet feeding pressure control unit thus provided, a substantially constant sheet feeding pressure may be applied even in a case where the weight of sheets varies depending on the sheet size, to thereby attain stable sheet feeding performance without causing double feeding and nonfeeding.
The conventional image forming apparatus including the sheet feeding pressure control unit as described above employs a single elastic member (pressure spring) as the pressing unit, in which the sheet feeding pressure is controlled by changing the acting length of the elastic member. However, the sheet feeding pressure control unit provided with a single elastic member is incapable of controlling a sheet feeding pressure according to, for example, the sheet size or the amount of the stacked sheets in order to maintain the constant sheet feeding pressure constant. FIG. 22A is a graph illustrating the weights of small-size sheets and large-size sheets applied onto the sheet stacking plate in relation to the number of stacked sheets. The graph assumes a case of employing portrait A4-size sheets and portrait A5-size sheets, which are commonly-used standard-size sheets, and the line L1 indicates a case of the portrait A4-size sheets while the line L2 indicates a case of the portrait A5-size sheets. FIG. 22B is a graph illustrating a relation between the number of stacked sheets and the sheet feeding pressure when the spring pressure (elastic force) of the elastic member is set so that a constant sheet feeding pressure may be applied regardless of the number of large-size sheets stacked on the sheet stacking plate. According to the graph of FIG. 22B, a sheet feeding pressure N1 is applied for large-size sheets while a sheet feeding pressure N2 is applied for small-size sheets. As illustrated in the graph, when the sheet feeding pressure N1 for large-size sheets remains constant, the sheet feeding pressure N2 for small-size sheets are high when the sheets are full-stacked.
Conventionally, the acting length of the elastic member is configured to be variable so that the acting length may be adjusted to be reduced when feeding small-size sheets, to thereby keep the sheet feeding pressure low for the small-size sheets. However, the sheet feeding pressure is exerted by only one elastic member, and hence the spring constant is always the same. Accordingly, even when the acting length is adjusted to be small, a rate of increase (gradient in the graph) in the sheet feeding pressure with respect to the number of stacked sheets is constant as indicated by the broken line N3 of FIG. 22B, and hence the sheet feeding pressure N3 may only be controlled to be in parallel with the sheet feeding pressure N2 in the graph. The sheet feeding pressure may not remain constant for small-size sheets from a less-stacked state to a full-stacked state.
When a single elastic member is employed as the pressing unit, a predetermined sheet feeding pressure fails to be exerted, causing the sheet feeding pressure to start dropping from a half-stacked state to a full-stacked state as indicated by N4 of FIG. 23. The loss in the sheet feeding pressure is ascribable to the sliding resistance of the sheet bundle on the sheet stacking plate against the regulating surface of the sheet feeding cassette and against the side regulating member, and to a reaction force generated by the stiffness of the sheet bundle. The loss in the sheet feeding pressure is more likely to be generated in the case where large-size sheets or elongated sheets are employed, and also becomes significant when the amount of stacked sheets becomes large. For example, when a setting is made so that a sufficient sheet feeding pressure may be exerted even in the full-stacked state of sheets which exhibits a high pressure loss, an excessive sheet feeding pressure is exerted in the less-stacked state as indicated by N5 of FIG. 23. In view of this, the acting length of the elastic member may be varied from the half-stacked state to the full-stacked state. In this case, however, the amount of stacked sheets needs to be detected to control, which scales up the apparatus.